Carbohydrates are a large class of natural substances that structurally are polyhydroxycarbonyl compounds and their derivatives. Carbohydrates generally correspond to the formula (C).sub.n (H.sub.2 O).sub.n, where n is an integer usually greater than 3.
Monosaccharides are simple carbohydrates that cannot be further hydrolyzed into simpler types of carbohydrate. A monosaccharide having a five-membered ring is referred to as a furanose. A furanose lacking one or more hydroxyl groups normally present in a carbohydrate is referred to as deoxy-furanose, with the carbon chain position at which the hydroxy is absent being indicated.
Azasugars are a class of saccharides in which the ring oxygen is replaced by an imino group (--NH--). A five-membered ring azasugar can be referred to as an azafuranose or a polyhydroxylated pyrrolidine compound, as well as being named as an aza derivative of an otherwise systematically or trivially named furanose monosaccharide.
As used herein, azasugars derived from pyrrolidines (azafuranoses) are hydroxylated at the 3 and 4 positions, have a hydroxymethyl group at the 5-position, and a methyl or hydroxymethyl at the 2-position, the 1 position being the nitrogen atom, in pyrrolidine nomenclature. A 2-hydroxymethyl group in pyrrolidine nomenclature corresponds to a 4-hydroxymethyl or 5-hydroxyl group in furanose nomenclature. Pyrrolidine nomenclature and numbering will usually be used herein, unless otherwise specified.
Azasugars and their derivatives have been identified as potent glycosidase inhibitors [Paulsen et al., Adv. Carbohydr. Chem. 1968, 23, 115; Fellows, Chem. Br. 1987, 23, 842; Truscheit et al., Angew. Chem. Int. Ed. Engl. 1981, 20, 744; Inouye et al., Tetrahedron 1968, 24, 2125; Muller, in Biotechnology, Rehm, H.-J. et al., eds., VCH Verlagsgesellschaft Weinheim 1985, Vol. 4, Chapter 18]. As such, azasugars can be useful for treating metabolic disorders such as diabetes [Liu, J. Org. Chem. 1987, 52, 4717; Bayer et al., Ger. Offen. DE 3620645; Anzeveno et al., J. Org. Chem. 1989, 54, 2539; Yoshikuni et al., J. Pharmacobio-Dyn 1988, 111, 356] or as antiviral agents [Karpas et al., Proc. Natl. Acad. Sci. 1988, .85, 9229; Walker et al., Proc Natl. Acad. Sci. 1987, 84, 8120; Winkler et al., J. Med. Chem. 1989, 32, 2084], as antimicrobial [Evans et al. J. Phytochemistry 1985, 24, 1953] and as anticancer agents [Humphries, M. J., et al. Cancer Res. 1986, 46, 5215].
Despite their clear usefulness, there is still a need for an effective synthesis of novel azasugars and their derivatives [Fleet, Chem, Br. 1989, 25, 287 and references cited therein; Bernotas et al., Tetrahedron Lett. 1985, 26, 1123; Setoi et al. Chem. Pharm. Bull. 1986, 34, 2642; Legler et al., Carbohydr.. Res, 1984, 128, 61; Kinast et al., Angew. Chem. Int. Ed. Engl. 1981, 20, 805; Hanesian, Chem. Ind. 2966, 2126; Schmidt et al., Justus Liebigs Ann. Chem. 2989, 5, 423; Pederson et al., Tetrahedron Lett, 1988, 29, 4645; Ziegler et al., Angew. Chem. Int. Ed. Engl. 1988, 29, 716; Buchanan et al., J. Chem. Soc. Perkin Trans. 1990, 699; Fleet et al., J. Chem. Soc. Perkin Trans. 1989, 665; Dondoni et al., J. Chem. Soc. Chem. Commun. 1990, 854; Fleet et al., Chem. Lett. 1986, 1051; Chen et al., Tetrahedron Lett. 1990, 31, 2229; Ciufolini et al., J. Am. Chem. Soc. 1989, 111, 3473].
Naturally occurring azasugars include 1-deoxynojirimycin (1,5-dideoxy-1,5-imino-D-glucitol), 1-deoxymannojirimycin (1,5-dideoxy-1,5-imino-D-mannitol), and castanospermine (1,6,7,8tetrahydroxyoctahydroindolizine). 1-Deoxynojirimycin was isolated from plants of the genus Morus [Yagi et al., Nippon Nogei Kagaku Kaishi 1976, 50, 5751; Vasella et al., Helv. Chim. Acta. 1982, 65, 1134] and from strains of Bacillus [Daigo et al., Chem. Pharm. Bull. 1986, 34, 2243]. 1-Deoxymannojirimycin was isolated from the legume Lonchocarpus [Fellows et al., J. C. S. Chem. Comm. 1979, 977]. Castanospermine is a plant alkaloid isolated from seeds of an Australian chestnut tree, Castanospermum australe [Saul et al. Arch. Biochem. Biophys. 1983, 221, 593]. Isolation of azasugars from nature is often expensive and time consuming. Therefore, several methods have been developed for the synthesis of these important compounds.
Both synthetic and semi-synthetic routes have been used in these syntheses [Inouye et al., Tetrahedron 1968, 24, 2125; Paulsen et al., Chem. Ber. 1967, 100, 802; Saeki et al., Chem. Pharm. Bull. 1968, 16, 2477; Paulsen et al., Adv. Carbohydr. Chem. 1968, 23, 115; Kinast et al., Angew. Chem. Int. Ed. Engl. 1981, 20, 805; Bernotas et al., Tetrahedron Lett, 1984, 25, 165; Bernotas, Tetrahedron Lett. 1985, 26, 1123; Setoi et al., Chem. Pharm. Bull. 1986, 34, 2642; Iida, J. Org. Chem. 1987, 52, 3337]. Natural sugars have been used as starting materials, but multiple protection and deprotection steps are required. For example, glucose was used in the synthesis of 1-deoxynojirimycin and 1-deoxymannojirimycin [Fleet, Chem. Br. 1989, 25, 287 and references cited therein; Bernotas et al., Tetrahedron Lett. 1985, 26, 1123; Chen et al., Tetrahedron Lett. 1990, 31, 2229].
An enzymatic synthesis based on fructose-1,6-diphosphate (FDP) aldolase (EC 4.1.2.3) has been recently developed and has proven to be a powerful approach for the synthesis of some azasugars [Pederson et al., Tetrahedron Lett. 1988, 29, 4645; von der Osten et al., J. Am. Chem. Soc. 1989, 111, 3924; Pederson et al., Heterocycles 1989, 28, 477; Ziegler et al., Angew. Chem. Int. Ed. Engl. 1988, 29., 716; Straub et al., J. Org. Chem. 1990, 55, 3926]. This enzymatic method involves the use of FDP aldolase from either rabbit muscle or Escherichia coli to catalyze the aldol condensation of dihydroxyacetone phosphate (DHAP; a donor substrate) with any of a number of possible omega-azidoaldehydes as a second, acceptor substrate [Durrwachter et al., J. Am. Chem. Soc. 1986, 108, 7812; Durrwachter et al. J. Org. Chem. 1988, 53, 4175; Pederson et al., Tetrahedron Lett. 1988, 29, 4645; Bednarski et al. Tetrahedron Lett. 1986, 27, 5807]. The omega-azidoketose phosphate produced is dephosphorylated and then reductively cyclized to form a deoxynojirimycin compound.
For instance, if DHAP is reacted with (RS)-3-azido-2-hydroxypropanal in the presence of FDP aldolase, the 2-epimers 1-deoxynojirimycin and 1-deoxymannojirimycin are produced upon catalytic reductive amination of the dephosphorylated enzyme reaction products [Pederson, R. L., et al. Tetrahedron Lett. 1988, 29, 4645].
After synthesis, 1-deoxynojirimycin can then be easily converted into castanospermine [Hamana et al., J. Org. Chem. 1987, 52, 5494]. The latter compound has been shown to inhibit the processing of the AIDS virus gp160 envelope protein precursor, and to modify the envelope glycoprotein, thus affecting the ability of the virus to enter cells [Walker et al., Proc. Natl. Acad. Sci. 1987, 84, 8120].
The synthesis of hydroxylated piperidines and pyrrolidines from dicarbonyl sugars via a one-step double reductive amination reaction has recently been reported. Reitz et al., Tetrahedron Lett., 1990, 31(47):6777. According to such a synthetic scheme, 2,5-anhydro-imino-D-glucitol and 1-deoxynojirimycin were prepared by reacting 5-keto-D-fructose and 5-keto-D-glucose, respectively, with benzhydrylamine in the presence of a borohydride reducing agent (NaCNBH.sub.3, MeOH).
Of the pyrrolidine-type azasugars reported, (2R,5R)-dihydroxymethyl-(3R,4R)-dihydroxypyrrolidine (Compound X, hereinafter) [Welter et al, Phytochemistry, 1976, 25, 747; Fleet et al., Tetrahedron Lett., 1985, 26, 1469; Shing, J. Chem. Soc. Chem. Commun., 1987, 262; Dureault et al., Synlett, 1991, 4, 225] showed a broad spectrum of inhibition against glycosidases, including .alpha.- and .beta.-glucosidases and .beta.-xylosidase [Fleet et al., Tetrahedron Lett. 1985, 26, 3127]. As there is no apparent structural similarity between that reported apparent structural similarity between that reported 2,5-disubstituted pyrrolidine and hexoses that are the natural enzyme substrates, the inhibition may be due to a different mode of binding.
It would therefore be of interest if one could examine the inhibition properties of diastereomers of Compound X and other derivatives. It would also be beneficial if the range of glycosidase enzymes inhibited by Compound X diastereomers and derivatives thereof could be broadened still further to include still other glycosidases. The disclosure that follows discusses such compounds.